I get the same but don't worry about it! Ignore that fixed 1920x1080 setting, it will always be there

. The box should switch to a 4k display (3840x2160) when a 4k video is played.
To test this, play some of the 4k videos on YouTube and switch the "Stats for nerds" option on. It will report the actual resolution in use at the time.
Accessing "Stats for nerds" on YouTube differs with the version of YouTube you have but for me (using YouTube for Anfdroid TV) it's this:
1. Start the video in YouTube
2. Switch OFF the mouse
3. Press the central OK button on the remote
4. Use the 'left arrow' key to highlight 'more options', press OK
5. The 'stats for nerds' icon is the last on the right, press OK
6. To switch it off, reverse the procedure.